1. Field of the Invention
The present invention relates generally to fluid vaporizing devices such as aerosol generators.
2. Brief Description of the Related Art
Aerosols are useful in a wide variety of applications. For example, it is often desirable to treat respiratory ailments with, or deliver drugs by means of, aerosol sprays of finely divided particles of liquid and/or solid, e.g., powder, medicaments, etc., which are inhaled into a patient""s lungs. Aerosols are also used for purposes such as providing desired scents to rooms, distributing insecticides and delivering paint and lubricant.
Various techniques are known for generating aerosols. For example, U.S. Pat. Nos. 4,811,731 and 4,627,432 disclose devices for administering medicaments to patients in which a capsule is pierced by a pin to release a medicament in powder form. A user then inhales the released medicament through an opening in the device. While such devices may be acceptable for use in delivering medicaments in powder form, they are not suited to delivering medicaments in liquid form. The devices are also, of course, not well-suited to delivery of medicaments to persons who might have difficulty in generating a sufficient flow of air through the device to properly inhale the medicaments, such as asthma sufferers. The devices are also not suited for delivery of materials in applications other than medicament delivery.
Another well-known technique for generating an aerosol involves the use of a manually operated pump which draws liquid from a reservoir and forces it through a small nozzle opening to form a fine spray. A disadvantage of such aerosol generators, at least in medicament delivery applications, is the difficulty of properly synchronizing inhalation with pumping. More importantly, however, because such aerosol generators tend to produce particles of large size, their use as inhalers is compromised because large particles tend to not penetrate deep into the lungs.
One of the more popular techniques for generating an aerosol including liquid or powder particles involves the use of a compressed propellant, often containing a chloro-fluoro-carbon (CFC) or methylchloroform, to entrain a material, usually by the Venturi principle. For example, inhalers containing compressed propellants such as compressed gas for entraining a medicament are often operated by depressing a button to release a short charge of the compressed propellant. The propellant entrains the medicament as the propellant flows over a reservoir of the medicament so that the propellant and the medicament can be inhaled by the user.
In propellant-based arrangements, however, a medicament may not be properly delivered to the patient""s lungs when it is necessary for the user to time the depression of an actuator such as a button with inhalation. Moreover, aerosols generated by propellant-based arrangements may have particles that are too large to ensure efficient and consistent deep lung penetration. Although propellant-based aerosol generators have wide application for uses such as antiperspirant and deodorant sprays and spray paint, their use is often limited because of the well-known adverse environmental effects of CFC""s and methylchloroform, which are among the most popular propellants used in aerosol generators of this type.
In drug delivery applications, it is typically desirable to provide an aerosol having average mass median particle diameters of less than 2 microns to facilitate deep lung penetration. Propellant based aerosol generators are incapable of generating aerosols having average mass median particle diameters less than 2 microns. It is also desirable, in certain drug delivery applications, to deliver medicaments at high flow rates, e.g., above 1 milligram per second. Some aerosol generators suited for drug delivery are incapable of delivering such high flow rates in the 0.2 to 2.0 micron size range.
Commonly owned U.S. Pat. Nos. 5,743,251 and 6,234,167, which are hereby incorporated by reference in their entireties, disclose aerosol generators, along with certain principles of operation and materials used in an aerosol generator, as well as methods of producing an aerosol, and an aerosol.
The invention provides a fluid vaporizing device that includes a capillary tube made from an electrically conductive material, with the capillary tube providing a passageway for a fluid. At least two electrodes are connected to the capillary tube, with a first one of the at least two electrodes being connected to the capillary tube closer to an inlet of the capillary tube than a second one of the at least two electrodes. The second electrode has an electrical resistance sufficient to cause heating of the electrode during use of the device, thereby minimizing heat loss at the outlet end of the capillary tube.
The invention also provides an aerosol generator that includes a capillary tube having an inlet end, and an outlet end. A first electrode is connected to the capillary tube and a second electrode is connected to the capillary tube, with the first electrode being closer to the inlet end than the second electrode. A voltage is applied between the first and second electrodes to heat a section of the capillary tube between the first and second electrodes, with the capillary tube being hotter at the second electrode than at the first electrode. The second electrode has sufficient electrical resistance to reach a temperature during application of the voltage between the first and second electrodes such that the temperature is hot enough to substantially prevent conduction of heat from the capillary tube to the second electrode.
The invention further provides a method of vaporizing a liquid in a capillary tube having an inlet, an outlet, and a heated section defined between an upstream electrode and a downstream electrode. The downstream electrode has an electrical resistance sufficient to cause heating of the downstream electrode during use of the device, thereby minimizing heat loss at the outlet end of the capillary tube, and both the upstream and downstream electrodes are electrically connected to the capillary tube. The method includes supplying liquid into the capillary tube through the inlet, and applying a voltage across the electrodes to generate heat in the heated section. The voltage also generates sufficient heat in the downstream electrode to substantially eliminate any significant temperature gradient between the downstream electrode and the capillary tube at the connection between the downstream electrode and the capillary tube.